This invention relates generally to reduction of fatigue in rotating machine components and, more particularly, to methods and apparatus for accurate and reliable measurements of rotor loading. The methods and apparatus are particularly useful for wind power generation but are not necessarily limited thereto.
Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Asymmetric loading across these rotors occurs due to vertical and horizontal wind shears, yaw misalignment, and turbulence. These asymmetric loads contribute to extreme loads and the quantity of fatigue cycles on the rotor blades and other wind turbine components.
Accurate measurement of rotor loads acting on electrical machines, such as wind turbines, is a necessary precursor to reduction of fatigue and extreme loads. Currently known methods for measurement of bending moment components of these loads at an upwind end of a main shaft are susceptible to zero-point offset shifts arising from sensor drift over time, sensor drift with temperature, and material deformations that occur in differing operational states. At least one known calibration strategy defines zero-point offsets in an idle operational state with a single yaw position, whereas control of asymmetric loads occurs during full and near full load operation with significant time delay, possible temperature change, far more thrust load and changing yaw orientation. The difference in operating state between system calibration events and active control events can result in false incorporation of offset shifts into the measurements of bending moment. Outcomes can range from sub-optimal load reduction to increased fatigue and extreme loads. Measurements can thus be susceptible to offset changes from several sources, including but not limited to unforeseen modes of mild deformation in each machine's bedplate.
The use of sensors to measure rotor loads as bending moment in the main shaft of a wind turbine is known. For example, see U.S. Pat. No. 7,160,083, entitled “Method and apparatus for wind turbine rotor load control,” issued to Pierce et al. and assigned to the assignee of the present invention or a related company.
Known embodiments of sensors measuring rotor loads are susceptible to offset errors in their home coordinate systems (i.e., offsets in the bending moments in the coordinate system with which the physical sensors rotate), arising from different operational states, thrust load, time, and/or temperature.
For example, strain gauges in blades have been used as sensors fixed to the Germanischer Lloyd (GL) chord coordinate system. Strain gauge pairs located 1.5 meters from each blade root have been used in measurement campaigns to calculate MYR, MZR, MYN (D) and MZN (Q) moments. This measurement technique requires two sets of coordinate transformations to arrive at fixed frame (MYN, MZN) bending moments. Relative zero-point offset errors in the strain gages can result in constant offsets to values in the rotating frame moments (MYR, MZR) which are in the home coordinate system of the aggregate set of sensors.